@nicola.fiore
As I know, we don't have a way to convert a Datastream data type to Eikon's fields, or vice versa.
For Eikon, you can use the Data Item Browser application to search for fields by using fields' descriptions.
For Datastream, you can use the Datastream Navigator to search for data types.
For WC05303 (Treasury Shares), the equivalent Eikon field could be TR.TreasurySharesCommonPrimaryIssue representing the number of common shares owned by the company itself and its consolidated subsidiaries.
However, it is better to directly contact the Eikon support team via MyRefinitiv to confirm it.